A Novel Analog Front End with Voltage-Dependent Input Impedance and Bandpass Amplification for Capacitive Biopotential Measurements

Figure 1. Configuration of analogue front ends (AFEs) involving bootstrapping techniques: (a) Previous AFE (BVFcf) bootstrapped by resistors Ra1 and Ra2, and a feedback capacitor Ca3. This serves as a voltage follower; (b) Proposed AFE (BNArf) bootstrapped by resistors Rb1 and Rb3, and a varistor with voltage-dependent resistance Rbv. This serves as a non-inverting bandpass amplifier by the use of resistors Rs and Rf, and capacitors Cs and Cf. A capacitor Cn is incorporated to reduce stray capacitance Cst.

Figure 2. Frequency-gain characteristics of the proposed AFE with bandpass amplification tuned separately for cECG and cEMG measurements. Maximum gains were set to 20 dB for both measurements. Frequency passbands were set to 0.5–103 Hz for cECG and to 20–513 Hz for cEMG, respectively.

Figure 3. Top view configuration of flexible sheet electrode (FSE) for cECG measurement. The configuration was modified from [27,45].

Figure 4. Schematic diagram of the measuring system for both cECG and cEMG. Total gain including AFE was set to 60 dB according to the type of AFE. The frequency bands achieved by a combination of DC suppression and low-pass filter (LPF) were set to 0.5–100 Hz for cECG measurement and to 20–500 Hz for cEMG measurement, respectively. The analog signal was digitized at 1000 Hz with 16-bit resolution between −10 and +10 V by the analog-to-digital converter.

Figure 5. Top view configuration of flexible pad electrode (left) and driven ground electrode (DGE, right) employed for cEMG measurement. The pad electrode has a five-layered structure. DGE is used for the negative feedback of a voltage synthesized from two voltages that were independently obtained with negative and positive electrodes in the pad.

Figure 6. The transition of cECG recordings measured through thick clothing (1.70 mm) in low-humidity environments. The previous AFE was used for the recordings of (a,c), whereas the proposed AFE was used for the recordings of (b,d). Both recordings (a,b) were measured from a healthy-weight-BMI subject (22.0, #A), and recordings (c,d) were from an overweight-BMI subject (28.6, #C).

Figure 7. Examples of transient response of cECG baseline to the onset of supine position of subject #A. Original cECG signals were measured using the (a) previous AFE and (b) proposed AFE, and then filtered to extract their baselines, respectively. The baseline recovery (BR) time is also indicated in each response.

Figure 8. Minute-by-minute transitions of the root mean square voltage (RMSVresp) extracted from cECG recordings measured with the previous AFE (blue) or the proposed AFE (green) in each subject. (a) Subject #A (BMI 22.0). (b) Subject #B (BMI 26.4). (c) Subject #C (BMI 28.6). (d) Subject #D (BMI 20.7).

Figure 9. Examples of simultaneously measured reference EMG (EMGref) and cEMG through cotton cloth (0.26 mm thick) under conditions of low-humidity (19%, 3.91 g/m3) from subject #E using (a) the previous AFE and (b) the proposed AFE.

Figure 10. Digital Fourier transform (DFT) spectra for cEMG recordings in Figure 9 obtained from subject #E using (a) the previous AFE and (b) the proposed AFE. The blue line corresponds to the spectra for “Elbow bended” segments, and the red line corresponds to that for “Elbow straightened” segments.

Figure 11. Comparison of the signal-to-noise ratio (SNR) of recorded cEMG between the previous and proposed AFEs: (a) Comparison for each subject, (b) Comparison for data gathered from all subjects.